Cytochemical and biochemical techniques are used to characterize membranes of the slime molds Didymium, Physarum, and Dictyostelium. Electron microscope observation of myxamoebae undergoing sexual fusion is planned, concentrating on surface changes which enable cells to fuse. Surface labeling with lectins is done in Dictyostelium, and biochemical characterization of myxamoebal membrane glycoproteins of Didymium is in progress.